The invention relates to an apparatus for monitoring the closed position of a locking gas cap, with a magnet disposed at or in the locking gas cap and a magnetic switch disposed at the tank connection pipe.
An apparatus of this type is described in the DE 44 04 014 A1 and serves to monitor the closed position of the locking gas cap of a motor vehicle so that, when the locking gas cap is not closed or closed incompletely, a warning signal appears on the dashboard of the vehicle.
In the case of the known apparatus, the locking gas cap has a bayonet catch and the magnet is disposed in such a manner, that it is in the vicinity of a reed switch, when the bayonet catch is locked. Depending on the construction, the reed switch is thus either open or closed when the locking gas cap reaches the locked position. Preferably, the reed switch is constructed so that it is closed in the locked position and that an associated evaluating circuit causes a warning signal to be displayed when the circuit of the reed switch is interrupted.
Since the magnetic field of the magnets cannot be localized to a narrow limited space, the position of the locking gas cap can be determined only relatively inaccurately according to this principle. For many tank caps, the locking gas cap is constructed as a screw-in plug. However, even in the case of bayonet-like tank caps, screw pitch surfaces are usually provided, which ensure that the plug, in the closed position, is pressed firmly against an associated seal. The possibility therefore exists that the locking gas cap is not turned completely into the end position, in which the tank opening is sealed reliably and in which a subsequent loosening of the locking gas cap due to vibrations is prevented because of the frictional engagement. However, because of the aforementioned inaccuracy in determining the position, the reed switch would also respond in such a case, so that the correct closing of the locking gas cap cannot be indicated reliably.